In a case where a user (for example, a teacher) teaches a viewer (for example, a student) by using image data of a plurality of pages as a teaching material, the user displays the plurality of pages individually on one display device. For example, the user displays image data of a first page on the display device to explain the image data of the first page. Then, the user displays image data of a second page on the display device to explain the image data of the second page. The viewer is thereby able to listen to the explanation on the image data of the first page while viewing the image data of the first page, and listen to the explanation on the image data of the second page while viewing the image data of the second page.
However, pages of image data to be displayed on the display device are switched one by one, so that it is difficult for the viewer to view the image data of the plurality of pages at once. That is, when displaying the image data of the second page on the display device, the user switches image data to be displayed on the display device from the image data of the first page to the image data of the second page, so that it is difficult for the viewer to view the image data of the first page and the second page at once.
Moreover, in a case where one display device is used as a touch panel display, a user displays image data of a first page on the display device to explain the image data of the first page, and inputs, to the display device, information (input information) indicating an explanatory note accompanying the image data of the first page. A viewer is thereby able to listen to the explanation on the image data of the first page while viewing the image data of the first page and the explanatory note. As above, it is desired that a viewer is able to view image data of several pages at once.
However, since pages of image data to be displayed on the display device are switched one by one, the input information input by the user is not reflected on image data to be displayed on the display device in some cases. That is, in a case where the explanatory note accompanying the image data of the first page has no relation with image data of a second page, the explanatory note accompanying the image data of the first page is not reflected on the image data of the second page. In this case, when switching image data to be displayed on the display device from the image data of the first page to the image data of the second page, the user is required to delete the explanatory note accompanying the image data of the first page from the display device. As above, it is desired that information (input information) input by a user is reflected on target image data.
Techniques of displaying image data on a plurality of display devices are disclosed in PTL 1 and 2.
In a technique described in PTL 1, when image data is displayed on a first display device, image data same as the image data that is displayed on the first display device is displayed on a second display device by using a screen capturer.
In a technique described in PTL 2, when a user inputs information (comment) to image data displayed on a first display device, image data (image data in which the comment is reflected) same as the image data displayed on the first display device is displayed on a second display device by using a screen capturer.